Hm...I'm not sure if what I've done was evil. I would call it diabolically clever. I'm not sure if I should tell you all, though...it's a special Online Chess (well, Chess IS a game, right?) technique that I thought of that pretty much guarantees a victory over your opponent.
I'm mentioning it because when I manage to pull this technique off, it fills me warm, guilty excitement (ewww), and I'm pretty sure the other guy (especially if he's a hardcore chess player) feels absolutely angry and dumbfounded.
And have I done anything evil in real life? Dunno, I usually never do anything severe (even in videogames, actually), and certainly nothing really worth mentioning here, after reading the caliber of sins that have been posted already. I guess the most evil thing I have done was stick a bunch of snails onto a firework (I was very, very young at the time).